1. Field of the Invention
This invention relates to a face pumped laser, and more particularly, to such a laser providing in one head a plurality of laser hosts allowing a greater volume of host material and therefore higher average power laser output while maintaining the benefits of the face pumped, total internal reflection laser technique.
2. Description of the Prior Art
It is well known in the art that laser devices emit electromagnetic radiation of wavelengths generally in the infrafred and visible portions of the electromagnetic spectrum. The radiation emission is substantially coherent and is characterized by a narrow wavelength band.
Laser devices are operable using a suitable medium or host in which a population inversion may be established in a particular metastable energy level by proper optical "pumping". Neodymium glass, ruby, helium, neon, and carbon dioxide are media which are commonly employed in lasers. By "pumping" or irradiating the active medium with electromagnetic radiation possessing power necessary to create the population inversion, conditions permitting coherent emission may be obtained. The means emitting the radiation causing inversion is called "pumping means" while the wavelength of the radiation is known as the "pumping wavelength".
Laser devices which emit high energy pulses are well known in the art as pulsed lasers. The emitted pulses, though high in energy, are short in duration, lasting on the order of milliseconds to picoseconds.
The laser output is only a small percentage of the input energy, a large percentage of the energy ending up as heat in the laser medium. In pulsed lasers, one or more surfaces of the active medium are cooled in order to extract the excess heat. Because of the limited thermal conductivity of the host material, the excess heat is more rapidly removed from near the surface than from the interior, causing the center of the host to be warmer than the surface. Repeated operation of the laser host, precluding a proper cooling period, causes a substantial thermal gradient to be established, resulting in possible strutural failure or distortion of the output beam. Lasers employing host materials characterized by low thermal conductivities are particularly limited to low pulse repetition rates.
Recent interest in high repetition rates of pulsed lasers has led to increased activity in this field. Since a number of materials, particularly those of the doped glass species, which are employed as the active laser host have low thermal conductivity, several approaches have been taken to overcome this restraint on pulse repetition rate. These approaches have in common the segmenting of a side-pumped, rod type laser host and interspersing of a flowing coolant. The result of this procedure is that the unwanted heat is more quickly removed through the relatively smaller dimension of the segmented laser host material and therefore the composite arrangement can be operated at higher pulse repetition rates without structural failure due to thermally induced stress.
The general form of these multi-segment rod type lasers as shown in U.S. Pat. No. 3,715,682 issued to Young have glass discs which are generally edge pumped and therefore have some radial thermal gradients due to non-uniform pumping. These radial gradients result in degradation of the beam coherence. Optical compensation can be introduced as shown in U.S. Pat. No. 3,675,152 issued to Young. The radial refractive index gradient compensation means utilized requires that thermal equilibrium be reached before the laser pulse has full energy and good coherence. An alternative optical compensation means is the multi-path array as shown in U.S. Pat. No. 3,628,179 issued to Cuff et al. This compensates for transverse temperature gradients due to coolant flow but does not compensate for the residual radial temperature gradient due to non-uniform pumping. All of the above multiple-segment rod type lasers have in common the limitation that the lasing energy passes through the coolant. When high energy and high repetition rates are employed, the lasing energy is partially absorbed by the coolant. This creates thermal gradients and thus index of refraction gradients which cause serious optical distortion or loss of coherence in the lasing beam.
Presently face pumped lasers are advantageously employed as pulsed lasers. A face pumped laser permits substantially uniform activitation and concomitant heating across the laser aperature in contract to the nonuniform optical properties of the side-pumped, rod-type laser device. U.S. Pat. No. 3,631,361--Almasi et al., assigned to the same assignee as the present invention, discloses an arrangement for substantially increasing the pulse repetition rate of face-pumped laser devices by eliminating the distortion due to nonuniform heating. The losses and the distortion due to the laser output beam passing through the interspersed coolant flow remain.
The face pumped laser with total internal reflection when uniformly activated has intrinsic beam wave front compensation for thermal gradients within the laser host and no wave front distorition due to laser output beam passage through the coolant. These characteristics are advantageously utilized for high average power, high beam intensity and high repetition rate lasers. Examples of this art are U.S. Pat. No. 3,633,126 issued to Martin et al and U.S. Pat. No. 4,214,216 issued to Jones, Jr., both assigned also to the same assignee as the present invention.
A unique characteristic of the face pumped laser with total internal reflection is that the laser beam is directed through the laser host by means of total internal reflections from the major faces which also admit the pumping radiation. Thus maintaining the optical alignment and, hence, the performance of a laser resonator that incorporates a face pumped laser host with total internal reflections is dependent upon maintaining the laser host major faces substantial flat and parallel. The potential means of disturbing the resonator alignment is to cause flexure of the laser host by either inadequate support or unequal heating (or cooling) of the major faces. Flexure of the laser host curves the major face where the total internal reflection occurs and cause the exiting laser beam to be non-parallel to the entering laser beam. Thus all faced pumped lasers with total internal reflections require proper means of supporting and cooling the laser host to ensure that there is no flexure of the laser host to cause misalignment that results in degraded performance.
A problem not solved by prior art for the face pumped laser with total internal reflection, however, is the provision, in one head, of a greater volume of laser host material without exceeding thermal stress limitations and without thermally inducing bending of the laser host body. The prior art for the face pumped laser with total internal reflection utilizes a single slab of laser host material wherein the vertical dimension, thickness, of the slab is limited by the stress induced by the thermal gradient from the hot center line to the cooled upper and lower surface of the host.
Consequently, a need exists for a face pumped laser with total internal reflection, e.g., the laser beam is entirely reflected off the two large opposed plane faces, in a zig-zag manner down the length of the slab, having a higher average power output but which retains nevertheless the benefits of the face pumped, total internal reflection laser technique.
It is, therefore, an object of the present invention to provide a greater volume of laser host material in a face pumped laser with total internal reflection so as to yield higher average power output.
It is a further object of the present invention to provide a multiple laser host body in a face pumped laser with total internal reflection which can yield higher average power output without increasing significantly the overall size of the laser.
It is a further object of the present invention to provide such a multiple laser host body in a face pumped laser with total internal reflection without exceeding the thermal stress limit in and without thermally inducing flexure or bending of the laser host body.
Other objects and advantages of the present invention will become apparent as the description thereof proceeds.